Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an inductively coupled RF plasma generating apparatus that is capable of providing both magnetic confinement and Faraday shielding.
Discussion of Related Art
Plasmas are used in a variety of ways in semiconductor processing to implant wafers or substrates with various dopants, to deposit or to etch thin films. Such processes involve the directional deposition or doping of ions on or beneath the surface of a target substrate. Other processes include plasma etching where the directionality of the etching species determines the quality of the trenches to be etched.
Generally, plasmas are generated by supplying energy to a neutral gas introduced into a chamber to form charged carriers which are implanted into the target substrate. For example, plasma doping (PLAD) systems are typically used when shallow junctions are required in the manufacture of semiconductor devices where lower ion implant energies confine the dopant ions near the surface of the wafer. In these situations, the depth of implantation is related to the bias voltage applied to the wafer. In particular, a wafer is positioned on a platen, which is biased at a negative potential with respect to the grounded plasma chamber. A gas containing the desired dopant materials is introduced into the plasma chamber. A plasma is generated by ionizing the gas atoms and/or molecules.
Once the plasma is generated, there exists a plasma sheath between the plasma and the surrounding surfaces, including the workpiece. The sheath is essentially a thin layer at the boundary of the plasma which has a greater density of positive ions (i.e., excess positive charge) as compared to the bulk plasma which is electrically neutral. The platen and substrate (e.g., wafer for doping applications) are then biased with a negative voltage in order to cause the ions from the plasma to cross the plasma sheath. During crossing of the sheath the ions acquire a kinetic energy equal with the potential drop across the sheath. Therefore the ions are implanted into the wafer at a depth proportional to the applied bias voltage. The ion dose implanted into the wafer determines the electrical characteristics of the implanted region and the uniformity of the dose across the wafer surface ensures that all devices on the semiconductor wafer have identical operating characteristics within specified limits. Each of these parameters are critical in the semiconductor fabrication process to ensure that all devices have the desired operating characteristics.
RF powered plasma sources can be capacitively coupled, inductively coupled or wave coupled (helicons). In capacitive coupling, the electrons in the plasma are accelerated directly by local electric fields generated at the surface of the electrodes by an RF power supply typically operating in the MHz range (0.4-160 MHz). Because the electric fields are oriented normal to the electrode surface they also accelerate ions that impact the electrode surface or a dielectric surface positioned in front of the electrode. Ion impact to the electrode or dielectric dissipates energy resulting in less energy for plasma generation. Moreover, ion impact to the electrode or dielectric causes an undesirable sputtering of the surface impacted. Sputtering is a process whereby atoms are ejected from a solid surface due to bombardment of the target by energetic particles. Capacitively coupled RF plasma sources also suffer from other disadvantages. For instance, the electrodes sometimes release unwanted impurities into the plasma. In addition, capacitively coupled RF plasma sources provide low plasma density therefore are less suitable for ion sources applications.
In inductive coupling, the plasma electrons are accelerated in a direction parallel to a current carrying antenna by an electric field resulting from an induced magnetic field according to the Maxwell-Faraday equation
      ∇          ×              E        ->              =      -                  ∂                  B          ->                            ∂        t            where, {right arrow over (E)} denotes electric field and {right arrow over (B)} is the magnetic field. The current in the antenna is generated by an RF power supply. Inductive coupling is more efficient than capacitive coupling since most of the coupled energy is dissipated through electron collisions with a neutral gas. A voltage proportional to the length and inductance of the antenna is developed across the antenna that induces a parasitic capacitive coupling to the plasma. Parasitic capacitance is an unwanted capacitance that can exist between two electronic components simply because of their proximity to each other. This creates the aforementioned undesirable additional power dissipation and material sputtering. However, the capacitive component can be suppressed by inserting a Faraday shield between the antenna and the plasma.
A Faraday shield is a device that is designed to block and focus electric fields. Such a Faraday shield may comprise an array of grounded conductors orthogonal to the antenna currents. The Faraday shield is designed to terminate the electric fields while allowing the magnetic fields to propagate.
Inductively coupled plasma generation configurations can be divided into two categories—those utilizing an internal antenna and those utilizing an external antenna. For internal antenna configurations the antenna (i.e., inductive coupler) is immersed into the plasma chamber traversing the chamber walls by way of localized vacuum feed-throughs. For external antenna configurations the antenna is positioned outside of the plasma chamber separated by a dielectric window.
It is advantageous to provide magnetic confinement to the inner surface of the plasma chamber to reduce plasma losses to the walls. This enables a higher plasma density driven by less RF power and further provides operation at lower neutral gas pressure as well as higher plasma uniformity. Magnetic confinement is typically achieved by distributing multi-cusp magnets just outside the plasma chamber walls. Internal antenna configurations allow better magnetic confinement than external antenna configurations but preclude the use of a Faraday shield. External antenna configurations place the antenna behind a dielectric window which interferes with the application of multi-cusp magnetic confinement on a significant portion of the plasma chamber surface area (i.e., the dielectric window).
Thus, a trade-off exists between internal and external antenna configurations in that an external antenna configurations allows the use of a Faraday shield inside the plasma chamber, but does not allow for magnets to provide plasma confinement and an internal antenna configuration allows the use of magnets for better plasma confinement, but does not provide for a Faraday shield.
Accordingly, the embodiments disclosed and claimed herein are an improvement to the art and describe a method and apparatus that provides both Faraday shielding and magnetic confinement for an inductively coupled RF plasma source.